Move sync testing doc into to ecosystem platform docs #25

Ie, move the contents of this doc into the new Ecosystem platform docs (aka dochub) here.

This is the first step in a team KR for Q1 2020 to Allow for complete Sync engine testing against staging.

The idea is that in the process of moving (and completely testing) these docs, I'll be able to pinpoint what's missing in our ability to test, and get the appropriate issues filed with relevant teams.

Since this likely will require getting those issues filed early in order to get them in for Q1, I'm prioritizing this one now.

Specifically, this needs to cover testing against staging for:

• Lockwise
• Fenix (not doing Fennec yet; by the team this is done, we'll ideally be moved over - if we need to come back to it, we will)
• iOS
• Desktop (test against OSX and Windows to make sure no difference that need to be called out)